Caption: Gas plumes on Enceladus. Cutaway computer illustration of plumes of water vapour and other gases escaping at high velocity from the south polar region of Saturn's moon Enceladus. The theory of a 'shear heating' mechanism suggests that tidal forces acting on fault lines in the moon's icy shell cause the sides of the faults to rub back and forth against each other. This produces enough heat to transform some of the ice into plumes of water vapour and ice crystals. Cold sub-surface ice (blue) becomes much warmer near the active fractures from which the plumes emanate.